Ethylene-vinyl alcohol (EVOH) is the result of hydrolysis of ethylene-vinyl acetate copolymer. Substantially fully hydrolyzed EVOH, that is with less than 2 or 3 mole percent remaining vinyl acetate, is an extrusible polymer which displays remarkable gas-barrier properties. The gas-barrier quality of EVOH is dependent upon the moisture content. Above a water content of about 7 or 8 percent, the oxygen permeability of EVOH rapidly increases.
Because EVOH is a relatively expensive, moisture sensitive polymer, often it is employed as a thin barrier layer within a multi-layer polymeric film or sheet structure. The structural layers usually are less expensive, moisture resistant polymers. Multi-layer films or sheets suitable for food packaging often have polyolefin or nylon structural layers on each side of the gas-barrier layer and for some applications have a heat-sealable surface layer of a low melting temperature material such as Surlyn ionomer, ethylene-vinyl acetate (EVA), ethylene-acrylic acid (EAA), or low-density polyethylene (LDPE). Because of the cost, dilution by blending EVOH with less expensive polymers such as polyolefins is desirable, but blending EVOH with other polymers usually increases the oxygen permeability.